t.Errorf("WriteAt(-10) = %v, %v; want 0, ...%q...", n, err, wantsub)
	}
}

// Verify that WriteAt doesn't work in append mode.
func TestWriteAtInAppendMode(t *testing.T) {
	defer chtmpdir(t)()
	f, err := OpenFile("write_at_in_append_mode.txt", O_APPEND|O_CREATE, 0666)
	if err != nil {
		t.Fatalf("OpenFile: %v", err)
	}
	defer f.Close()

	_, err = f.WriteAt([]byte(""), 1)

	}
	n, err = lw.w.Write(p)
	lw.remain -= n
	return n, err
}

func TestMarshalWriteErrors(t *testing.T) {
	var buf bytes.Buffer
	const writeCap = 1024
	w := &limitedBytesWriter{&buf, writeCap}
	enc := NewEncoder(w)
	var err error
	var i int
	const n = 4000
	for i = 1; i <= n; i++ {
		err = enc.Encode(&Passenger{
			Name:   []string{"Alice", "Bob"},

	if cap(buf) < len(buf)+20 {
		b := make([]byte, len(buf), 20+cap(buf)*2)
		copy(b, buf)
		buf = b
	}
	writeAt := len(buf)
	buf = buf[0 : len(buf)+ctxt.Arch.PtrSize]
	writePtr(ctxt, buf[writeAt:], word)
	return buf
}

// Write a pointer-sized uint to the beginning of buf.
func writePtr(ctxt *obj.Link, buf []byte, word uint64) {